It's possible to upgrade a Joomla 1.5 installation to a 2.5 or 3.0 version of Joomla. After doing this you will find that the database you end up with it totally different to the usual 2.5 installation. Consequently the installation contains a mixture of 2.5 and 1.5 tables.
The jupgraded 2.5 database also seems to contain other differences when compared with a standrd Joomla 2.5 installation. This means that it's not possible to port one jupgrade database from one Joomla installation to another.